The present invention relates to a dynamic pressure bearing, and more particularly to a dynamic pressure bearing that prevents leaking and splashing of a magnetic fluid and resolves the problems caused by an instability phenomenon of fluid pumping resulting from an inbalance in dynamic pressure.
In the prior art, dynamic pressure bearings have been known in which a thrust plate is fixed to an end of a shaft, a thrust dynamic pressure bearing section is formed by the axial face of the thrust plate and the opposite face of a protruding tubular section of a fitted sleeve, a radial dynamic pressure bearing section is formed by the outer peripheral face of the shaft and the opposite face of the protruding tubular section of the fitted sleeve, a lubricating oil is retained in the dynamic pressure bearing sections, and a dynamic pressure is generated by the rotation of a rotor. In such dynamic pressure bearings, the lubricating oil filling the dynamic pressure bearing sections has to be effectively prevented from leaking and splashing. Furthermore, because the lubricating oil, which is the medium of the magnetic fluid, evaporates into the atmosphere as time passes, the service life of the bearings is limited or they required maintenance such as refilling. Another problem associated with dynamic pressure bearings is that the rotation of a sleeve or the like is unstable due to an unstable flow of magnetic fluid caused by pumping originating in the dynamic pressure bearing sections. This unstable flow of magnetic fluid caused by pumping originates because the dynamic pressure generated in the dynamic pressure bearing is unbalanced in the upper and lower axial directions and the magnetic fluid flows in a certain single direction.
Devices to prevent the lubricating oil from leaking or splashing in which the end portions of magnetic fluid are magnetically sealed with seal gap sections by forming a magnetic circuit using magnetic materials and magnetic fluid are known (for example, see Patent Documents 1 and 2). However, those devices require a strong magnetic flux to hold the two end portions of the magnetic fluid and a magnetic circuit incorporating a powerful permanent magnet has to be formed inside the sleeve and shaft.
A taper seal using the balance between the surface tension and atmospheric pressure is also known (for example, see Patent Document 3). However, the taper seal has a function as a reservoir to reserve the lubricating oil, while maintaining a capillary phenomenon by enlarging the height of the tapered section gradually, therefore, for the taper seal to use the capillary phenomenon, the gap to retain the lubricating oil has to be narrow. Particularly, in dynamic pressure bearings with spindle motors used in 3.5-inch hard disk drives for computers, flexibility in designing the shape of the taper seal is strictly limited and a high machining accuracy based on mirror finishing or the like is required. As a result, the production cost of components is increased. Furthermore, in order to increase the accumulation capacity, the length of the tapered section has to be elongated because the taper angle cannot be enlarged enough, but this causes problems because the degree of freedom in designing the shape of the taper seal is restricted.    Patent Document 1: Japanese Patent Publication No. H6-33941 (Page 3, FIG. 1, and FIG. 3)    Patent Document 2: U.S. Pat. No. 4,694,213 Specification (Pages 3 to 5, FIG. 1)    Patent Document 3: Japanese Patent Publication No. 2001-112214 (Pages 4 and 5, FIG. 1)